I have always wondered, why do stars twinkle?

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Stars twinkle because of their great distance from the Earth. Any given star is so far away that its light is literally a point in the sky. All of the light from that star comes through the Earth’s atmosphere and to our eyes in exactly the same direction and is bent in exactly the same way by turbulence in the air. The star will “disappear” briefly when its light is bent away from our eyes, then reappear when its light is not bent off course on its way to our eyes.

By the way, planets in our Solar System, because they are much, much closer to the Earth, appear larger and do not appear as points. Their light travels to our eyes along many different paths and is not all simultaneously bent away from our eyes by atmospheric turbulence, and consequently they do not twinkle.